Resolution of chiral molecules is required in many areas of research. As enzymes and other biological receptor molecules are stereo-specific, enantiomers of a racemic compound may interact with them in a different manner. Consequently, two enantiomers of a racemic compound have different pharmacological activities in many instances. In order to discern these differing effects, the biological activity of each enantiomer needs to be studied separately. This has contributed significantly towards the requirement of enantiomerically pure compounds particularly in pharmaceutical industry and thereby the needs to focus on chiral separation using techniques like chiral chromatography. Various attempts have been made in the past for the development of different stationary phases; for example A. Bielejewska et al. Chem. Anal. (Warsaw) 47 (2002) 419 has reported β-cyclodextrin (β-CD) and permethylated β-cyclodextrin for use of chromatographic separation of mandelic acid and its esters of different aliphatic carbon chain length by reverse phase HPLC. The drawbacks of this process are; (i) β-cyclodextrin alone does not recognize enantiomers of mandelic acid; (ii) stationary phase needs to be permethylated for achieving high chiral separation; (iii) reaction has to be conducted in reverse phase.
S. P. Mendez et al. J. Anal. At. Spectrom. 13 (1998) 893. reported the resolution of D,L-selenomethionine derivatives of OPA (O-phthalaldehyde) and NDA (2,3-naphthalenedicarboxaldehyde) to their respective enantiomers by HPLC on a β-CD chiral column using conventional fluorimetric detection. The drawbacks of this process are; (i) In this study, the amino acid was derivatized using o-phthalaldehyde or naphthalene-2,3-dicarboxaldehyde to allow conventional fluorimetric detection. Such a derivatization step, however, is undesirable because it prolongs the sample preparation time, and requires additional validation because it may be a potential source of contamination, may induce racemization or may complicate the separation.
L. S. Karen et al. Analyst 125 (2000) 281 disclosed the work based on a commercially available HPLC column with a chiral crown ether based stationary phase to perform enantiomeric separations of selenoamino acids without derivatization. The drawbacks of this process are; (i) the need to have dilute perchloric acid as mobile phase for such a column; (ii) The separation of the enantiomers is temperature sensitive.
C. A. L. Ponce de Leon et al. J. Anal. At. Spectrom. 15 (2000) 1103 describes the enantiomeric separation of nine selenoamino acids encountered in selenium-enriched yeast using a crown ether column. The drawbacks of this process are; (i) this reaction involves acidic condition to get effective separation; (ii) The separation process requires lower temperature (18-22° C.) for complete resolution; (iii) the non-polar amino acids may not elute from the column, therefore, a balance between temperature and elution of non-polar compounds is required for an optimum separation.
S. P. Mendez et al. J. Anal. At. Spectrom. 15 (2000) 1109 described the use of teicoplanin-bonded chiral stationary phase (Chirobiotic T) to resolve a variety of underivatized aminoacids. Teicoplanin is a glycopeptide antibiotic which contains 20 chiral centers. The drawbacks of this process are; (i) Teicoplanin is a toxic and naturally occurring complex molecule therefore cannot be easily tuned for various applications (ii) due to the presence of many glycosidic linkages it is prone to hydrolysis and/or alteration in conformation thereby change in optical properties under the elution conditions (iii) this separation process requires pH adjustment about 4 and 7; (iv) separation has to be conducted in reverse phase.
M. Raimondo et al. Chem. Commun. (1997) 1343 used mesoporous silica-based MCM-41 coated on GC capillary columns, as chiral stationary phase to separate different organic molecules The drawbacks of this process are; (i) That separation indeed occurs within the MCM-41 cavities and by a mechanism depending on the proton affinities of the compounds.
M. Grun et al. J. Chromatogr. A 740 (1996) 1 described the behavior of silica, alumina, titania, zirconia and the novel mesoporous aluminosilicate MCM-41 in normal-phase high-performance liquid chromatography under comparable conditions. MCM-41 shows some interesting features as compared to mesoporous crystalline and amorphous oxides. The drawbacks of this process are; (i) This work includes only comparison of an ordered mesoporous aluminosilicate, silica, alumina, titania and zirconia in normal-phase high-performance liquid chromatography; (ii) it requires very large column (250×4 mm).
V. A. Soloshonok, Angew. Chem., Int. Ed. 45 (2006) 766), reported the work based on achiral silica as column packing material for remarkable separation of enantiomers of perfluoroalkyl keto compounds through column chromatography. The drawbacks of this process are; (i) only trifluoromethyl group containing compounds are separated. (ii) variation in results is found with changing the solvents. (iii) In the case of preferential homochiral association, the situation is bit subtle as the formation of dimer will result in different number of enantiomeric (S)(S) and (R)(R) pairs with identical scalar properties. These dimers therefore cannot be separated.
J. H. Kennedy, J. Chromatogr. A 725 (1996) 219 disclosed chiral stationary phases based on polysaccharide derivative coated on silica for chiral separation of different compounds containing carbonyl group and other aromatic ring containing compounds. The drawbacks of this process are; (i) Derivatization of carboxylic acids or eluent modifiers such as acetic acid or diethyl amine is required in this system; (ii) Polysaccharide phases based chiral stationary phase is not predictable and capable of separating both t-acid and n-basic type compounds.
X. Huang et al. Analytical Science 21 (2005) 253 and S. Rogozhin et al. German Patent 1 932 190 (1969); Chem. Abstr., 72 (1970) 90875c have described the use of chiral copper metal complex supported on silica as stationary phase for separation DL-selenomethionine in buffered solution at pH, 5.5 along with methanol as mobile phase. The drawbacks of this process is (i) This separation technique requires 200×4.6 mm i.d. stainless-steel column; (ii) only underivatized amino acids were resolved on it; (iii) the use of methanol doesn't favor the resolution of DL-selenomethionine; (iv) higher temperature gives some de-activation effect of some biological sample.
J. Bergmann et al. Anal. Bioanal. Chem. 378 (2004) 1624 and M. M. Bayon et al. J. Anal. At. Spectrom. 16(9) (2001) 945 disclosed a fast and sensitive method for the determination of the absolute configuration of Se-amino acids by derivatization process at room temperature by reversed-phase high-performance liquid chromatography-inductively coupled plasma-mass spectrometry. The drawbacks of these process are; (i) separation can be possible in reversed phase HPLC— inductively coupled plasma-mass spectrometry; (ii) Detection limits of about 4 microg L(−1) were obtained; (iii) The derivetization of enantiomers of selenomethionine is necessary. (iv) The final operating conditions involved the use of 50% (v/v) MeOH at pH 5.3 (acetic acid-sodium acetate).
H. Kosugi et al. Chem. Commun. (1997) 1857 described synthesis of (−)-epibatidine and its intermediates by medium pressure liquid chromatography by using achiral silica gel column (Si-10; eluted with 3:1 hexane-EtOAc; UV (254 nm) and RI detectors). The drawbacks of this process are; (i) In this system there is no mechanism of the separation: (ii) It includes only synthesis of (−)-epibatidine and its intermediates; (iii) only hydroxy acetal was separated through achiral column chromatography.
S. P. Mendez et al. J. Anal. At. Spectrom. 14 (1999) 1333 described chiral resolution and speciation of DL-selenomethionine enantiomers by capillary gas chromatography (GC) using an L-valine-tert-butylamide modified polydimethylsiloxane as chiral stationary phase The drawbacks of this process are; (i) good resolution was achieved in the higher temperature range only from 100-160° C.; (ii) requires He as carrier gas; (iii) separation is more difficult for complex biological samples.
R. Vespalec et al. Anal. Chem. 67 (1995) 3223; K. L. Sutton et al. Analyst 125 (2000) 231; S. P. Mendez et al. Anal. Chim. Acta 416 (2000) 1; J. A. Day et al. J. Anal. At. Spectrom. 17 (2002) 27 describes capillary electrophoresis as a tool for the enantiomeric separation selenium containing amino acids, by derivatization process using capillary electrophoresis with UV absorbance detection. The drawbacks of this process are; (i) This separation technique has been used to separate the enantiomers of selenoamino acids by the addition of chiral additives to the electrophoretic buffer; (ii) UV absorbance detection was used in these studies and required the derivatization of the selenoamino acids to permit detection; (iii) UV absorbance detection, without sample pre-concentration, was not sensitive enough to permit the detection of the low levels of selenoamino acids present in complex samples; (iv) applied voltage and pH value gives variation in separation results; (v) buffer system was chosen for good resolution; (vi) addition of methanol to the buffer is required for improved resolution.
B. V. Ernholt et al. Eur. J. Chem. 6 (2000) 278) described the synthesis and enzymatic separation of 1-Azafagomine through achiral regular column chromatography. The drawbacks of this process are; (i) enzymetic separation requires different buffer solutions; (ii) the conversion and enantiomeric excess is affected by varying the solvents, enzymes and its concentration; (iii) low enantiomeric excess was achieved through achiral column chromatography by loading 51% compound.
A. Goswami et al., Z Tetrahedron Asymmetry, 16 (2005) 1715 disclosed enzymatic separation of (±)-sec-butlylamine, lipase and proteases using ether, heptane or dacane as solvent and vinyl butyrate or ethyl butyrate as acylating agent. The drawbacks of this process are; (i) enzymes shows very low enantio-selectivity; (ii) it's a time consuming process (more than 7 days); (iii) solvent, such as acetonitrile, cyclohexane, toluene, methyl-t-butyl ether, 2-methyl-2-pentanol, ethyl caprate is required for this system. Mitsuhashi Kazuya et at in U.S. Pat. No. 278,268 Oct. 23, 2002 disclosed a method for the synthesis of optically active mandelic acid derivatives by enzymatic separation. The drawbacks of this process are; (i) microorganism is essential to generate the (R)-5-mandelic acid derivative or (S)-mandelic acid derivative; (ii) requires appropriate buffer solution.
Mod Takao et al. U.S. Pat. No. 142,914 Oct. 29, 1993 disclosed a process for preparing D-mandelic acid by converting L-mandelic acid into benzoylformic acid followed by stereoselectively reducing it into D-mandelic acid. The drawbacks of this process are; (i) The isolation and collection of microbial cells from culture broth is complicated; (ii) buffer solution is required for maintaining pH; (iii) it is time consuming process.
Endo Takakazu et at in U.S. Pat. No. 677,175 Mar., 29, 1991 disclosed process for producing (R)-(−)-mandelic acid or a derivative through enzymatic separation. The drawbacks of this process are; (i) hydrolysis of mandelonitrile is necessary; (ii) requires neutral or basic reaction system to produce the (R)-(−)-mandelic acid; (iii) requires expensive use of microorganism. and Ghisalba Oreste et at in U.S. Pat. No. 360,802 Jun. 2, 1989 described process for the preparation of R- or S-2-hydroxy-4-phenylbutyric acid in very high enantiomeric purity by enzymatic separation. Disadvantage of this process are; (i) The reduction of the substrate is effected by the so-called final reductase; (ii) suitable as biocatalysts are only purified enzymes; (iii) regeneration of enzyme is complicated.
Hashimoto Yoshihiro et al in U.S. Pat. No. 764,295 Dec. 12, 1996, reported a process for producing an alpha-hydroxy acid or an alpha-hydroxyamide from an aldehyde and prussic acid with a microorganism. The drawbacks of this process are; (i) deactivation of microorganism within a short period of time at higher and lower temperature; (ii) high concentration and high yield is difficult to obtain for alpha-hydroxy acid or alpha-hydroxyamide; (iii) the reaction rate is lowered with an increase in the concentration of the alpha-hydroxy acid or alpha-hydroxyamide product as a result, the reaction does not proceed to completion.
Endo Takakazu et al in U.S. Pat. No. 904,335 Jun. 25, 1992 described a process for producing (R),(S)-mandelic acid or a derivative thereof from mandelonitrile using a microorganism belonging to the genus Rhodococcus. The drawbacks of this process are; (i) chiral reagents and microorganism are more expensive; (ii) this method is industrially non-advantageous for producing (R)-(−)-mandelic acid or derivatives; (iii) hydrogenases produced by these bacteria are not always satisfactory.
R. Charles et al. J. Chromatogr. 298 (1984) 516 described the separation of 14C labelled nicotine through totally achiral column chromatography. The drawbacks of this process are; (i) it requires buffer solution to adjust the pH; (ii) peak-splitting phenomenon was caused by some components of the cation-exchange column or mobile phase.
V. A. Soloshonok et al. J. Fluorine Chemistry, In Press disclosed the self-disproportionation chromatography (SDC) involves the separation of trifluoromethyl group containing compounds and used totally achiral silica as column packing. The drawbacks of this process are; (i) variations in results are found with changing the solvents. (ii) In the case of preferential homochiral association, the situation is bit subtle as the formation of dimmer will result in different number of enantiomeric (S)(S) and (R)(R) pairs with identical scalar properties. These dimers therefore cannot be separated.